1. Field of the Invention
The present invention relates to a sheet feeding device for feeding sheets loaded in sequence one at a time.
2. Related Background Art
FIGS. 12 to 14 show a conventional sheet feeding device.
The sheet feeding device includes a sheet feeding shaft 201 which is rotatably supported by right and left bearings (not shown) of a device body. A sheet feeding roller 202 is fitted on the sheet feeding shaft 201 with a cylindrical core 203 formed integrally with the sheet feeding roller 202 therebetween. The sheet feeding roller 202 and the cylindrical core 203 are rotatable relative to the sheet feed shaft 201 but not movable along the sheet feeding shaft 201. The sheet feeding roller 202 and the cylindrical core are made of, for example, a rubber.
A clutch gear 209 is fitted on the sheet feeding shaft 201 at the right end portion thereof, as viewed in FIGS. 12 to 14, in such a manner as to be rotatable relative to the shaft 201 but not movable along the shaft 201. A driving gear 211 driven by a motor (not shown) meshes with the clutch gear 209 so as to allow the rotational force in a counterclockwise direction indicated by an arrow `a` in FIG. 12 to be transmitted to the clutch gear 209.
A spring winding drum 209a is formed integrally with and coaxially with respect to the clutch gear 209. A spring winding drum 207a is coaxially fixed to the sheet feeding shaft 201 adjacent to the spring winding drum 209a. Both the spring winding drums 209a and 207a have substantially the same diameter. A clutch spring 219a is coiled over the spring winding drums 201 and 207a, and a one-revolution control ring 206a is loosely fitted on the clutch spring 219a. One end portion 291c of the clutch spring 219a is locked to the spring winding drum 207a, and the other end portion 219d is locked to the control ring 206a. The control ring 206a has on its outer surface a claw portion 206b, which can be engaged with and disengaged from a first claw portion 212a.
The spring winding drums 209a and 207a, the clutch spring 219a, the control ring 206a and the claw portion 206b in combination constitute a known one-revolution spring clutch (hereinafter referred to as "a first spring clutch B").
The cylindrical core 203 of the sheet feeding roller 202 extends from the right end portion of the sheet feeding roller 202. A spring winding drum 207b is coaxially fixed to the sheet feeding shaft 201 adjacent to the right end portion of the extending portion of the cylindrical core. The extending portion of the cylindrical core 203 and the spring winding drum 207b have substantially the same diameter. A clutch spring 219b is coiled over both the extending portion of the cylindrical core 203 and the spring winding drum 207b, and a control ring 208a is loosely fitted over the clutch spring 219a. One end portion 219c of the clutch spring 219b is locked to the extending portion of the cylindrical core 203, and the other portion 219d thereof is locked to the control ring 208a. The control ring 208 has claws 208b formed on the entire outer periphery thereof at a small pitch. A second claw portion 212b can be engaged with and disengaged from the claws 208b.
The cylindrical core 203, the spring winding drum 207b, the clutch spring 219b, the control ring 208a and the claw portions 208b in combination form a known spring clutch (hereinafter referred to as a second spring clutch C).
A cam 204 is fixed to the sheet feeding shaft 201 between the first and second spring clutches B and C.
A flapper 212 is attracted to and released from a solenoid 213. The flapper 212 has on its distal end side the first claw portion 212a which can be engaged with and disengaged from the claw portion 206b of the control ring 206a of the first spring clutch B, and the second claw portion 212b which can be engaged with and disengaged from the claw portions 208b of the control ring 208a of the second spring clutch C.
A sheet loading base 214 is disposed with a distal end side thereof being located below the sheet feeding roller 202. The sheet loading base 214 is urged by a pushing spring 216 in a direction in which the distal end side thereof approaches the under surface of the sheet feeding roller 202. Sheets S are loaded on the loading base 214.
A friction pad 215 for separating one sheet from the others is disposed downstream of and adjacent to the distal end portion of the sheet loading base 214 in a state in which it is in contact with the under surface of the sheet feeding roller 202 by a spring 217.
When sheets are not fed out, the solenoid 213 is off (deenergized), and the flapper 212 is pulled down by a spring (not shown) with its first and second claw portions 212a and 212b respectively engaged with the claw portions 206b and 208b of the control rings 206a and 208a of the first and second spring clutches B and C (the state indicated by the solid line in FIG. 13). Consequently, the control rings 206a and 208a are checked, and the first and second spring clutches B and C are thus off.
That is, in the first spring clutch B, the clutch spring 219a is loosely held on the spring winding drums 209a and 207a (in a clutch-off state). Consequently, the clutch gear 209 rotated by the driving gear 211 idles over the sheet feeding shaft 201, and no driving force is thus transmitted to the sheet feed shaft 201, i.e., the shaft 201 is maintained in non-rotating state.
In the second spring clutch C, the sheet feed shaft 201, i.e., the spring winding drum 207b, is not rotated. Also, the clutch spring 219b is loosely held (in the clutch-off state) on the spring winding drum 207b and the extending portion of the cylindrical core 203 of the sheet feeding roller 202, and the sheet feeding roller 202 is thereby maintained in a non-rotating state.
The cam 204 is positioned at a rotational angle at which the large-diameter portion thereof is directed downward. Consequently, the distal end of the large-diameter portion of the cam 204 is in contact with the upper surface of the sheet loading base 214, and the sheet loading base 214 is thereby pressed down to a predetermined pressed down position (FIG. 13) against the spring 216. In this state, the upper surface of the distal end portion of the sheets S loaded on the loading base 214 is separate from the under surface of the sheet feeding roller 202 by a distance `.alpha.`.
When the solenoid 213 is turned off (energized) on the basis of a sheet feed starting signal, the flapper 212 is attracted to the solenoid 213, and the first and second claw portions 212a and 212b of the flapper 212 are respectively disengaged from the claw portions 206b and 208b of the control rings 206a and 208a of the first and second spring clutches B and C (the state indicated by the dot-dot-dashed line in FIGS. 12 and 13). Consequently, the control rings 206a and 208b are released, and the first and second spring clutches B and C are thereby turned on.
More specifically, in the first spring clutch B, when the control ring 206a is released, the clutch spring 219a is tightened on the spring winding drums 209a and 207a, and the clutch gear 209 which is rotating by the rotation of the driving gear 211 is thereby connected to the sheet feed shaft 201 through the spring winding drum 209a, the clutch spring 219a and the spring winding drum 207a, thereby rotating the sheet feed shaft 201 together with the clutch gear 209 in a direction indicated by `a`.
In the second spring clutch C, as the control ring 208a is released, the clutch spring 219b is tightened on the spring winding drum 207b and the extending portion of the cylindrical core 203 of the sheet feed roller 202. Consequently, the spring winding drums 207b and the extending portion of the cylindrical core 203 are connected to each other, and the sheet feed roller 202 thereby starts rotating together with the sheet feed shaft 201 which has started rotating by the turning on of the first clutch spring B in a direction indicated by `a`.
As the sheet feed shaft 201 rotates, the cam 204 formed integrally with the sheet feed shaft 201 also rotates, rotating the downward large-diameter portion thereof in a direction in which it is moved away from the upper surface of the sheet loading base 214. At the initial stage of the rotation of the large-diameter portion, pressing down of the sheet loading base 214 by the cam 204 is cancelled, and the sheet loading base 214 is thereby pushed up by the spring 216, bringing the upper surface of the distal end portion of the sheets A loaded on the sheet loading base 214 into contact with the under surface of the sheet feed roller 202 (FIG. 14).
Therefore, a feeding force is applied to the sheet located on the top of the sheet pile S loaded on the base 214 by the sheet feed roller 202 which has started rotating, and only the sheet located on the top of the pile is separated from the other sheets by the friction pad 215, and is fed out of the sheet loading base 214 between the sheet feeding roller 202 and the friction pad 215.
Thereafter, the sheet is further conveyed by register rollers 218 to a sheet receiving portion of an image formation unit. Between the sheet feed roller 202 and the register rollers 218, the sheet is guided by a sheet guide plate.
After the sheet has been fed out by the rotation of the sheet feed roller 202 and then accepted by the register roller 218 (within a time required for the sheet feed roller 202 to make one rotation), the solenoid 213 is turned off. Consequently, the flapper 212 is released from the solenoid 213, and the first and second claw portions 212a and 212b of the flapper 212 are respectively thereby moved down toward the control rings 206a and 208a of the first and second spring clutches B and C.
As a result, in the second spring clutch C, the claw portion 212b of the flapper 212 immediately comes into engagement with the claw portion 208b of the control ring 208a, thereby checking the control ring 208a and turning the clutch off. As a result, the sheet feed roller 202 is disconnected from the sheet feed shaft 201 and is thereby made free from the rotation of the sheet feed shaft 201.
In the first spring clutch B, after the control ring 206a has made one rotation, the claw portion 206b thereof is brought into engagement with the first claw portion 212a of the flapper 212 which is moving down toward the claw portion 206b, thereby checking the control ring 206a. As a result, the clutch is turned off, and the rotation of the sheet feed shaft 201 stops. That is, one-rotation drive of the sheet feed shaft 201 ceases. At that time, one rotation of the cam 204 also ceases and the cam 204 returns to its position where the large-diameter portion is directed downward. Therefore, the sheet loading base 214 is pressed down against the spring 216, and the sheets S loaded on the base 214 are separated from the sheet feed roller 202 by the distance `a`.
Convey of the sheet continues due to the conveying force of the register rollers 218 even after the first and second spring clutches B and C have turned off. Since the cylindrical core 203 has been disconnected from the sheet feed shaft 201 by the turning off of the second spring clutch C and the sheet feed roller 202 has thereby been made free from the rotation of the sheet feed shaft 201, the sheet feed roller 202 and cylindrical core 203 are rotated over the shaft 201 by the conveying force of the register rollers 218 until the rear end of the sheet passes between the sheet feed roller 202 and the friction pad 215.
Among the two spring clutch mechanism B and C employed in the above-described sheet feed device, the first spring clutch mechanism B for rotating the cam 204 to move sheet loading base 214 up and down must have very accurate dimensions with respect to the spring winding drum and clutch spring. Furthermore, the assembly of the spring clutch mechanisms requires troublesome tasks, including coating of a grease and adjustment of the backlash of the winding drum in the thrust direction. These increase production cost.
FIGS. 15 and 16 show another conventional sheet feeding device.
A sheet feeding device shown in FIGS. 15 and 16 includes a sheet feed roller 250 made of a friction member, a driving shaft 215 for driving the sheet feed roller 250, and a spring clutch 256 mounted on one end of the driving shaft 251. The spring clutch 256 consists of a gear 252, a control ring 253, a spring 254 and a boss 255.
The driving shaft 251 is supported by a support plate 257 of an apparatus body through a bearing 258. The gear 252 is rotated by a drive force transmitted thereto from a drive source. The control ring 253 has on its outer peripheral surface a claw portion 253a which can be locked by an actuator (not shown) of a solenoid. The boss 255 is fixed to the driving shaft 251 by means of a vis 259.
The spring 254 is wound around both a ring portion 252a of the gear 252 and a ring portion 255a of the boss 255 in a direction in which the spring 254 tightens up on the ring portion 252a due to friction when the gear 252 is driven in a direction indicated by an arrow `A`.
When the gear 252 is rotated in the direction indicated by the arrow `A`, the spring 254 thus tightens up on the ring portion 252a, allowing the drive force to be transmitted to the sheet feed roller 250 through the boss 255 and driving shaft 251. One end 254a of the spring 254 is locked to a groove portion 255b of the boss 255, and the other 254b thereof is locked to a notch portion 253b of the control ring 253.
Therefore, when the claw portion 253a of the control ring 253 is locked to the actuator, even if the gear 252 is rotated, the spring 254 is fitted loosely over the ring portion 252a, allowing the gear 252 alone to rotate with its ring portion 252a sliding against the spring 254.
As a result, rotation of the sheet feed roller 250 can be controlled by operating the actuator which is achieved by turning on and off of the solenoid (not shown).
However, the aforementioned conventional sheet feed device has the following drawbacks.
The ring portion 252a of the gear 252 must be made of a sintered material because of sliding of the spring 254 thereagainst. Also, the ring portion 252a must be coated with a lubricant oil. These increase production cost.
Furthermore, when transmission of the driving force is suspended by locking the claw portion 253a of the control ring 253 to the actuator, a load may be applied to the spring 254, thereby generating noises.
Furthermore, an idling torque is generated even while the driving force is not being transmitted. This may apply an excess load to the driving source.